This invention relates to a process for making silicon carbide whiskers and is particularly concerned with a process for providing relatively high yields of beta silicon carbide whiskers.
Silicon carbide is a high strength ceramic material which has good chemical stability and excellent oxidation resistance to high temperatures due mainly to the fact that it is covalently bonded and has a crystal structure related to that of diamond. Silicon carbide whiskers are an elongated form of silicon carbide having an aspect ratio, i.e., a length-to-diameter ratio, greater than about 3 and a typical diameter between about 0.1 and 10 microns. The high aspect ratio of whiskers makes them a much more effective reinforcement in composites than silicon carbide particulates. Also, whiskers exhibit much higher mechanical strength than silicon carbide fibers, which are either polycrystalline or amorphous forms of silicon carbide that typically have a diameter greater than 10 microns. These superior properties of silicon carbide whiskers have led to their use as a reinforcing material for ceramics, metals, polymers and glass composites. Silicon carbide whiskers are particularly suited for use in the reinforcement of all types of engineering ceramics including gas turbine ceramics, automotive ceramics and ceramic cutting tools. It is estimated that the market for the use of whiskers as reinforcements for engineering ceramics alone will be several hundred million dollars per year by the year 2000.
The current dominant technology for producing silicon carbide whiskers is the carbothermal reduction of silica by carbon in rice hulls. Rice hulls are composed of about 15 to 20 percent ash that is primarily silica. Thus, when the rice hulls are retorted or pyrolyzed in an inert atmosphere at a temperature in the vicinity of 1800.degree. C. while removing gases and other vapors as they form, carbon in the rice hulls reacts with silica to form silicon carbide in accordance with the overall reaction of EQU 3C+SiO.sub.2 =SiC+2CO(g).
The major problem with using rice hulls to synthesize silicon carbide whiskers is that the resultant product normally contains only between about 10 and 20 weight percent silicon carbide whiskers with the remainder being silicon carbide in the form of particulates, unreacted silica and unreacted carbon. Higher yields of silicon carbide whiskers are usually not possible because the chemical composition of the rice hulls and the degree of mixing of the carbon and silica therein are set by nature and cannot readily be varied. Thus, it is difficult to obtain an intimate mixture of carbon and silica that is sufficiently porous to allow carbon monoxide gas to escape and thereby drive the overall reaction of carbon with silica to form silicon carbide to completion while allowing space for silicon carbide whiskers to grow.
Accordingly, it is one of the objects of the present invention to provide a process for producing silicon carbide in which the silica and carbon originate from different sources, which sources are chosen to obtain an intimate mixture which results in the more efficient production of greater amounts of silicon carbide whiskers. It is another object of the invention to provide a process in which the size and shape of the whiskers can be varied depending upon the ultimate application for the whiskers. These and other objects of the invention will become more apparent in view of the following description of the invention.